The present invention relates to a gasoline engine having direct injection.
In direct injection gasoline engines, a combustion chamber in each cylinder is delimited by a longitudinally movable piston and the inside wall of a cylinder head, with an injector injecting fuel into the combustion chamber to form a mixture internally with combustion air supplied separately. The composition of the fuel/air mixture must be within the ignitable window in order to be ignitable by a spark which can be triggered between the electrodes of a spark plug.
German Published Patent Application No. 195 46 945 describes a direct injection internal combustion engine in which injectors with their injection nozzles inject fuel in a conical pattern into the combustion chamber, the spark plug being arranged in such a way that its electrodes are outside the lateral surface of the cone of fuel produced by the injection nozzle. This prevents fuel from wetting the electrodes during the injection operation and counteracts deposits of carbon on the electrodes due to incompletely burned fuel. The electrodes are free of coking over a long period of operation, which should thus guarantee proper functioning of the internal combustion engine without misfiring. To introduce an ignitable mixture between the electrodes arranged outside the cone of fuel, the spark plug is arranged so that the ground electrode is a short distance away from the lateral surface of the fuel cone and the inside wall of the cylinder head extends parallel to the lateral surface of the fuel cone, thus forming an interspace at least at the location where the electrodes of the spark plug are arranged.
A turbulent flow composed of the fuel/air mixture and extending into the area of the electrodes should be formed in this interspace. To generate a turbulent flow, a special shaping of the inside wall and arrangement of the spark plug near the injector are necessary. The injector is arranged in a counterbored hole in the inside wall, i.e., it is set back from the free combustion chamber volume, so that the vortex of mixture should be formed in the area adjacent to the injection nozzle and should circulate in the hollow space formed in the area of the injection nozzle between the lateral surface of the fuel cone and the inside wall of the cylinder head. In addition, air displaced by fuel injected into the combustion chamber should flow back through the air gap between the fuel cone and the parallel inside wall of the cylinder head, which is also conical. Additional fuel particles should be entrained from the fuel cone during the flow back to the spark plug along the inside wall. Turbulent flow is formed in the area near the injector to a great enough extent to introduce an ignitable mixture between the electrodes of a spark plug. The spark plug must therefore be arranged close to the injector.
With a conventional direct injection gasoline engine, the perimeter of the combustion chamber must be formed precisely and at great expense by the inside wall of the cylinder head in particular in order to achieve the desired hydrodynamic effects to form the ignitable mixture vortex. The conventional combustion chamber configuration having the combustion chamber shape required to form a mixture vortex and having the spark plug necessarily arranged near the injector often does not yield an optimum combustion and cannot guarantee the desired operating performance of the internal combustion engine.
It is an object of the present invention to provide a direct injection gasoline engine in which the engine will operate at an optimum operating performance.
In the case of the combustion chamber configuration according to the present invention, the fuel cone is injected in a free jet which is substantially unaffected by the perimeter of the combustion chamber, i.e., the cone of fuel is injected at such a great distance in particular from the inside wall of the cylinder head, that the conical fuel jet spreads out in the free volume of the combustion chamber substantially without hydromechanical wall effects of the combustion chamber perimeter. In injection, vortices of fuel are formed, emerging from the lateral surface of the cone and composed at first mainly of fuel vapors mixing with the surrounding combustion air in the combustion chamber. Fuel vortices develop to an especially great extent when the cone angle of the fuel cone is between 70xc2x0 and 100xc2x0, and they are produced by an air flow developing in the area of the lateral surface of the fuel cone due to the air entrained by the fuel jet, with an air flow also being produced in the opposite direction due to the resulting vacuum. The spark plug is positioned according to the present invention so that the electrodes project into the fuel vortex of the free jet. The spark position of the electrodes may be 1 mm to 15 mm from the lateral surface of the fuel cone.
The fuel vortex which brings ignitable mixture between the electrodes is formed on the lateral surface of the free jet without any effective influence on the part of the perimeter of the combustion chamber, so that the shape of the combustion chamber may be configured freely. This is a jet-guided combustion method in which wall effects of the inside wall of the cylinder head or a piston recess have hardly any effect on formation of the mixture or on ignition. Especially in stratified charge operation of an internal combustion engine when operating with fuel injection during the compression stroke and with the combustion chamber filled with air, a central cloud of fuel is formed so that optimum combustion of the combustion chamber charge may be achieved with a simple combustion chamber configuration. Another advantage of the formation of the mixture according to the present invention is that the spark plug may be arranged at a greater distance from the injector. The fuel vortex is then at the same location in the combustion chamber for a longer period of time, so that ignition may occur independently of the injection time over a wide interval.
The free jet of fuel may be injected into the combustion chamber in the form of a hollow cone. In this way, the fuel vortices develop in a form that is particularly suitable for transporting the mixture toward the spark plug, particularly in the case of injection under a high cylinder pressure in the compression phase during stratified charge operation. It is particularly simple to produce a cone of fuel having a hollow conical shape if the injector has an injection nozzle that opens outwardly.
Fuel vortices develop out of the fuel cone in different zones on the lateral surface as a function of the load and the rpm of the internal combustion engine. To obtain the most optimum spark position in the fuel vortex in ignition and thereby improve ignition and complete combustion of the mixture in the combustion chamber, at least two electrodes to which the positive potential of the ignition voltage may be applied are provided at different spark positions relative to the lateral surface of the fuel cone. According to the prevailing operating load and/or rpm of the internal combustion engine, the spark is triggered between the optimally positioned electrode having a positive potential relative to the fuel vortex and the respective ground electrode.
To guarantee theoretically optimal combustion of the fuel/air mixture, an infinitely adjustable spark position relative to the edge vortex as a function of load and rpm is necessary. With several electrodes to which the positive potential of the ignition voltage may be applied, theoretically optimal ignition of the edge vortex may be approximated if the electrodes are arranged at different spark positions relative to the lateral surface of the fuel cone. In the case of an arrangement of two electrodes to which the positive potential may be applied, the electrodes may be arranged so that the possible lateral surface zone in which fuel vortices are manifested is completely covered with the corresponding spark positions.
The electrodes having a positive potential may be arranged at different depths of penetration in the combustion chamber. For reliable coverage of the fuel vortices in different load and rpm ranges through ignition in the optimum spark position, it may be advantageous to arrange the electrodes having a positive potential at different distances from a central axis of the cylinder. The electrodes having a positive potential may have different alignments of their longitudinal axes relative to the central axis of the cylinder.
Depending on the mode of operation (stratified charge operation/formation of a homogeneous mixture) of the internal combustion engine and the operating conditions, the ignition voltage is applied using a control unit between either one of the electrodes for the positive potential and the respective ground electrodes. Ignition then occurs through the electrode which is in an optimum spark position relative to the existing fuel vortex. A second retarded firing (double firing) may be provided through the additional electrode to support complete combustion of the charge. Simultaneous ignition in several spark positions may also be possible.
In an example embodiment of the present invention, two spark plugs are provided in different positions relative to the fuel jet, each spark plug having a ground electrode and an electrode with a positive potential. The positive potential of the ignition voltage is applied to the central electrode. Ignition in different selectable spark positions as a function of load and rpm may also be accomplished through a single spark plug having electrodes of different axial lengths to which the positive potential of the ignition voltage may be applied.